Printing plates



acterized by an environment of hydrogen ion United States Patent C PRINTING PLATES Charles H. Van Dusen, Jr., Timherlake, Ohio, assignor to Addressograph-Multigraph Corporation, Wilmington, DeL, a corporation of Delaware No Drawing. Application May 17, 1951, Serial No. 226,935

2 Claims. (Cl. 101-149.2)

This invention relates to lithographic printing plates, and more particularly to a desensitized metal plate and method of producing the same.

Historically, the art of lithography was based on the 'discovery that certain stones possessed oleophilic as well as hydrophilic properties and that this inherent characteristic could be utilized in a new method of printing. Accordingly, a greasy image was delineated on the surface of the stone, and this step was followed by a water treatment in the form of a spray or light swipes with a damp cloth. When an inking roller was subsequently moved across the surface of the stone, the film of water standing in the non-design areas thereof rejected or repelled the ink; on the other hand, the design areas defined by the greasy image picked ink oit the roller. The stone was then ready for duplication simply by pressing the sam against a blank copy sheet.

As far as the fundamental principle is concerned, present-day practice differs little from the earliest method. However, the cumbersome nature of the stones soon dictated that a substitute lithographic surface be found, and for this answer the art turned to zinc and aluminum plates. It was soon realized that the numerous tiny pits and crevices in the surface of the original stone provided tiny reservoirs for the water film as well as sharp apices which functioned to anchor the greasy image, and of course it was highly desirable to have the same general type of surface on the new metal plates. This was accomplished by pitting the surface of the new metal plate either by chemical etchants, or by mechanical graining and pitting means such as abrasive buffing. Since the present invention is not limited by any particular method of graining and pitting, and since these treatments are well known in the art, it is not deemed necessary to dwell on this point.

In order to obtain a higher degree of image definition on the metal plates, a light-sensitive colloid, such as bichromatized albumin, is conventionally coated on the pitted and grained metal surface, this coating process being termed sensitizing. When the sensitizer used is given an exposure through a transparency, it is differentially hardened, and by washing out the exposed plate with water and a desensitizing solution, the non-hardened portions of the sensitizer are washed away, but the lighthardened portions are unaffected thereby and cling to the metal surface to define the printing areas thereof.

It is always absolutely essential, when working with metal plates mme form of zinc or aluminum foils, that only the light-hardened portions representing the image areas remain on the plate after the desensitizing treatment, for if any albumin or equivalent sensitizer representing non-design areas is left behind, these areas will tend to pick up ink during use. Obviously such a condition is highly undesirable, for the albumin scum, as it has been termed, tends to pick up ink and imperfect prints result. In an attempt to counter-act scumming in metal plates, the art has evolved certain desensitizing solutions, chard s It 2,732,796 C Patented Jan. 31 1 956 radicals, for removing the undeveloped sensitizer from the non-design areas. However, the plates so treated still exhibit a marked tendency toward scumming, probably due to the fact that such desensitizing procedures are mostly acid in nature; in fact, the scum is so tenacious at times as to require the plate to be remade. Also, when working with imaged zinc and aluminum plates, the art has experienced a major problem in the tendency for these plates to corrode during storage.

It is therefore an object of this invention to eliminate albumin scumming, and to reduce toning in metallic lithographic printing plates to a point where any toning that does appear can be readily removed by conventional means.

More specifically it is an object of this invention to produce a metallic plate or foil for use in lithography which shows no tendency to accumulate sensitizer scum or to tone up during use.

It is a further object of this invention to provide a lithographic printing plate from which all traces of unhardened albumin or the like can be positively removed from the plates or foils in the first instance without dependence upon any separate chemical desensitizing procedure.

Still a further object of this invention is to provide a novel method of treating lithographic printing plates prior to their being sensitized, said' method being characterized by a desensitizing treatment in which the solution used is alkaline in nature in contradistinction to the acidified desensitizing solutions heretofore employed in the art.

Yet a further object of the present invention is to provide a new and improved pitted and grained zinc or aluminum foil for use in lithographic printing methods.

Still a further object of the invention is to provide a plate that exhibits substantially no tendency to corrode during prolonged periods of storage.

In contrast to the platesor foils of the prior art and the partially-effective desensitizing methods, I have discovered that if zinc or aluminum plates or foils, having conventional pitted and grained surfaces, are given a treatment with a solution of silicate of the alkali class, such plates exhibit no tendency whatsoever toward scumming, and toning tendency is substantially reduced. This treatment with an alkaline solution is quite in contrast to previous desensitizing treatments wherein an acid solution was invariably chosen in order to counteract scumming tendency. Thus, the plate of the present invention is characterized, broadly speaking, by a tough, extremely tenacious and difiicult-to-remove, insoluble silicate film immediately overlying and in contact with the pitted and grained surface of zinc and aluminum plates or foils. A plate having the type of surface mentioned is simply dipped in, or wiped with, a solution similar to those hereinafter to be disclosed, rinsed in water and allowed to dry. The solution can be applied hot or cold. The plate is then ready for a coating of sensitizer, no deoxidizing treatment being required, and following exposure, the non-hardened sensitizer is readily and completely removed with a simple water treatment. Plates so treated exhibit no tendency to scum, due to the completely effective removal of the unhardened albumin or equivalent sensitizer, and observations during printing runs have shown that toning tendency is substantially reduced.

One formulation for the silicate solution of the present invention which I have found to be entirely satisfactory is as follows:

This to ulati 31 mercial solution of sodium silicate supplied by the Philadelphia Quartz Company and known as N grade, the analysis of which shows 37.6% solids and 62.4% water. One part by weight of*N grade sodium silicatewas diluted with 6.52-par.tsby weight ofwaten to give. Formulation I, and it will be noted that the solids content isaapproximately This alkaline solution was applied at room temperature to an aluminum foilwhich. had been pitted and grained in aconventional manner, after which the plate was air-dried, sensitized Withlight-sensitiVe albumin and treated with developing inkin a conventional manner developed with water, inked and run. in a rotary lithographic press in the usual manner. While the particular type ofaluminum plate islargely a-matter of. choice, I have found the 2S and 35 commercial. gra'desto-be. entirelysatisfactory.

Another typical formulation for the silicate treating.

solutionof the present inventionis as follows:

II Component: Parts by weight N320 9 SiOz 6. H2O 300 This solution was similarly applied to a zinc foil, which also had been pitted and grained in a conventional manner. The zinc plate treated with Formula II gave results similar in every respect to the aluminum plate of Formula I. The solids content of this solution represents a commercial grade of sodium silicate supplied by the Philadelphia Quartz Company and known as Metso 99, and it will be noted that the solution itself is also 5% by weight. However, my tests show that the per cent of solids may be varied throughout a relatively wide range. For instance, Formula I may range as low as 0.1% with satisfactory results. Further, the alkali oxide-silicon dioxide ratio may be varied from 3:2 to 123.22. It is not essential that sodium silicate be used, for I have found that potassium silicate works equally well. Thus, a 5% solution of KzSiOa in which the mol ratio of K20 to SiOz was 1:39 gave equally good results with both zinc and aluminum plates which had been pitted and grained by anyof the conventional methods.

The practice of the present invention can be used with both zinc and aluminum plates with equally good results, and it is not limited by the specific manner of pitting and graining the metal surface; While I prefer to use albumin as the sensitizer, experimentation has shown that the type of sensitizer used makes no difference asfar as prevention of scumming and toning is concerned. For instance, the plate of the present invention is equally capable of producing similar results with light-sensitized gela tine, fish glue, and the like. Sensitizers as a class, as Well as their theory of operation, are well known in the art, and since the present invention is not confined to any particular one, no further explanation in this respect is deemed necessary, other than to add that they are generally recognized as light-sensitizable hydrophilic colloids. It is also to be understood that the foregoing examples are illustrative only, viscosity, proportions and equivalents being adjusted and selected for the particular need and balance as desired.

While the present invention is by no means to be limited to any particular theory, I believe that the film on the surface of the plate is the result of'the metal in the plate combining with the soluble, alkali silicate to form an insoluble silicate. In any event, the film is something more than a mere silica coating, for it is water insoluble,

remove the same; but at the same time it does not interfere with plate sensitizing treatments, nor does it impair image-retention.

From the foregoing, it will be seen that the present invention provides a plate that is pre-desensitized in nature, requiring no deoxidizing and-only a simple water treatment for removal of the unexposed sensitizer, and one which undergoes no corrosion-during storage. The presenceof the silicate film displacesthe need for. conventional desensitizing solutions, counter-etching; and deoxidizing procedures heretofore found necessary, and in addition provides a plate which does not scum. Aside from the novel method itself, the plate' of. thepresent invention is characterized by a conventionalpitted and grained aluminum or zinc lithographic surface whichlsurface has tenaciously adherent thereto a thin film of silicate. Finally, it will be observed that I am treating zinc and aluminum plates for use in lithography with a desensitizing solution or coating material that is alkaline in nature, whereas previous desensitizing practice has been restricted to an acid treatment in this respect.

Hence, while I have illustrated and described the preferred embodiments of my invention, it is to be understood that these are capable of variation and modification, and I therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall within the purview of the following claims.

I claim:

1. In a metallic lithographic printing plate selected from the group consisting of. zinc and aluminum having a pitted and grained obverse surface to provide water-retaining reservoirs and image-retaining apices, a thin waterinsoluble film united 'with an overlying completely said surface configuration and which consists of an insoluble silicate that is the reaction product of the metal of the plate and a water soluble alkali silicate selected from the group consisting of sodium and potassium, and which film is afforded by treating the pitted and grained surface of the plate with a water solution of said soluble alkali silicate in which the alkali silicate concentration lies within the range of from 0.1% to 5% by Weight and the excess of which is rinsed away with water so as to leave on said surface only said reaction product in film form of molecular dimension preserving the essential pitted and grained surface configuration of the plate.

2. A method for preparing lithographic printing plates selected from the group consisting of zinc and aluminum having a pitted and grained obverseside comprising, treating the entire area of such side of the plate with a water solution of an alkali silicate selected from the group consisting of sodium and potassium to establish on the plate a thin adherent and water-insoluble film which is the water-insoluble reaction. product of the metal in the plate and said alkali silicate, the amount of silicate in said solution being within the range of from 0.1% to 5% by weight, and then rinsing in water the plate thus treated and allowing the same to dry.

References Cited in the file of this patent UNITED STATES PATENTS 1,557,354 Galetzka Oct. 13, 1925 1,574,378 Dobinson Feb. 23, 1926 1,634,658 Flammer et a1. July 5, 1927 2,132,443 Simons Oct. 11, 1938 2,156,100 Simons Apr. 25, 1939 2,186,946 Wood Jan. 16, 1940 2,225,736 Champion et a1. Dec. 24, 1940 2,233,573 Ayers Mar. 4, 1941 2,280,986" Toland et al. Apr. 28, 1942 2,297,929 Wise Oct. 6, 1942 

1. IN A METALLIC LITHOGRAPHIC PRINTING PLATE SELECTED FROM THE GROUP CONSISTING OF ZINC AND ALUMINUM HAVING A PITTED AND GRAINED OBVERSE SURFACE TO PROVIDE WATER-RETAINING RESERVOIRS AND IMAGE-RETAINING APICES, A THIN WATERINSOLUBLE FILM UNITED WITH AN OVERLYING COMPLETELY SAID SURFACE CONFIGURATION AND WHICH CONSISTS OF AN INSOLUBLE SILICATE THAT IS THE REACTION PRODUCT OF THE METAL OF THE PLATE AND A WATER SOLUBLE ALKALI SILICATE SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, AND WHICH FILM IS AFFORDED BY TREATING THE PITTED AND GRAINED SURFACE OF THE PLATE WITH WATER SOLUTION OF SAID SOLUBLE ALKALI SILICATE IN WHICH THE ALKALI SILICATE CONCENTRATION LIES WITHIN THE RANGE OF FROM 0.1% TO 5% BY WEIGHT AND THE EXCESS OF WHICH IS RINSED AWAY WITH WATER SO AS TO LEAVE ON SAID SURFACE ONLY SAID REACTION PRODUCT IN FILM FORM OF MOLECULAR DIMENSION PRESERVING THE ESSENTIAL PITTED AND GRAINED SURFACE CONFIGURATION OF THE PLATE. 